Semiosis (from Ancient Greek σημείωσις (sēmeíōsis), from σημειῶ (sēmeiô) 'to mark'), or sign process, is any form of activity, conduct, or process that involves signs, including the production of meaning. A sign is anything that communicates a meaning, that is not the sign itself, to the interpreter of the sign. The meaning can be intentional such as a word uttered with a specific meaning, or unintentional, such as a symptom being a sign of a particular medical condition. Signs can communicate through any of the senses, visual, auditory, tactile, olfactory, or taste.

The term was introduced by Charles Sanders Peirce (1839–1914) to describe a process that interprets signs as referring to their objects, as described in his theory of sign relations, or semiotics. Other theories of sign processes are sometimes carried out under the heading of semiology, following on the work of Ferdinand de Saussure (1857–1913).